Thin film sensors are already known, in particular for measuring the degree of humidity of a gaseous medium. These sensors are essentially constituted by a smooth support which is generally made of glass, on which there are deposited a first film of metal, then a polymer film, then a second film of metal which is very thin (150 .ANG.) so as to be porous and sensitive to water vapor. The polymer acts as a dielectric in a capacitor and is made of a hydrophilic material which is thin (10 microns) in order to ensure that the sensor's response time is short, e.g. about one second.
The properties looked for in such a sensor are obtained by accurately making thin films of constant thickness. This causes both the cost and the reject rate to be high since the polymer film is often too thin and thus forms a short circuit between the electrodes.
The state of the art is illustrated in French published patent specification No. 2 204 520. Practical fabrication difficulties are also explained in another French published patent specification No. 2 339 169 in which proposals are made to use a hydrophilic material comprising one or more monomolecular films.
Preferred implementations of the present invention provide a method of fabricating a short response time thin film sensor in which the dielectric is made more easily than before, and may even be made on an irregular base. In addition, such implementations of the invention do not suffer from a high reject rate.
As a result of such simpler and easier fabrication, sensors can be made more cheaply than before.